Learn the distance of the strong stab, which is very important. ONLY ever use strong stab, which is right click (secondary fire if you've modded your controls). Stand next to a wall and see the farthest distance you can be while still hitting the wall with the strong stab. Once you've got that down, it's about the timing, and stab as soon as your opponent is within that range. Also, no certain location does a noticable difference in damage, except the back. One strong stab to the back will be a 1 hit kill, regardless of armor or HP. Never jump in a knife fight either, that tends to make you a sitting duck for enough time so that the other guy can get a free shot. And where to aim. I'd say aim at the middle of their chest, since its a large target, and it's easier to hit. Also, instead of trying to beat your opponent to the first hit (if you both have 100 hp) brush against them and make certain that you'll get your first hit, and THEN use the knife range distance thing I just explained. Basically make sure you get that first stab in, and then play it safe after that. However, that's just the way I do things, because normally I can get the second stab in before the other knifer can. It depends on your style, and wether or not you like to wait around a corner, or just have a free-for-all in the open.
Hope that helps :thumbup:
and sorry for the long post, but there's some decent info in there